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Abstract:

A method of manufacturing oriented Si steel containing Cu with high
electric-magnetic property comprises: hot rolling slab; after first cold
rolling, heating it to 800° C. or higher temperature and
performing intermediate decarburization annealing in a protective
atmosphere with P.sub.H2O/P.sub.H2 of 0.50˜0.88 for 3-8 minutes, to
decrease carbon content of the steel plate to less than 30 ppm; then
peening and acid-pickling to remove oxide of Fe on surface and to control
oxygen content to lower than 500 ppm; secondary cold rolling to final
thickness and coating separation agent in water-slurry form; drying to
decrease water content to lower than 1.5%; high-temperature annealing in
a protective atmosphere containing hydrogen with oxidation degree
(P.sub.H2O/P.sub.H2) of 0.0001-0.2; finally applying a tension coating
and leveling tension annealing.

Claims:

1. A method for producing grain-oriented silicon steel containing copper,
comprising: Secondary refining and continuous casting of molten steel in
a converter or an electric furnace to obtain casting blank having the
following composition based on weight: C 0.010%-0.050%, Si 2.5%-4.0%, Mn
0.1%-0.30%, Als 0.006%-0.030%, Cu 0.4%-0.7%, N 0.006%-0.012%,
S≦0.025%, balanced by Fe and unavailable inclusions; Hot rolling,
acid washing, primary cold rolling, degreasing and middle decarburizing
annealing, wherein the middle decarburizing annealing is carried out by
heating the steel sheet to 800.degree. C. or higher in a protective
atmosphere with P.sub.H2O/P.sub.H2=0.50-0.88 for 8 minutes or shorter to
reduce the carbon content in the steel sheet to 30 ppm or less; Shot
blasting and acid washing for removing iron oxides from the surface to
control the oxygen content to be 500 ppm or less; Acid washing and
secondary cold rolling for rolling the steel sheet to desired thickness;
High-temperature annealing; and Applying tension coating on the surface
of the steel sheet and stretch-leveling annealing.

2. The method of claim 1 for producing grain-oriented silicon steel
containing copper, wherein in the high-temperature annealing process, the
steel sheet is coated with a high-temperature annealing separator in the
form of aqueous slurry after the secondary cold rolling and dried to
reduce the water content of the separator to less than 1.5%, or dry
coated directly by electrostatic coating; and the steel sheet is
high-temperature annealed in a protective atmosphere comprising hydrogen,
wherein the oxidability (P.sub.H2O/P.sub.H2) of the protective atmosphere
is in the range of 0.0001-0.2.

3. The method of claim 2 for producing grain-oriented silicon steel
containing copper, wherein the main component of the high-temperature
annealing separator is selected from any one of zirconia ceramic fine
powder, alumina fine powder and silicon dioxide fine powder, or a
combination of any two or three of zirconia ceramic fine powder, alumina
fine powder and silicon dioxide fine powder.

[0002] Currently, the developmental trend of the processes for producing
grain-oriented silicon steel is directed to heating of slab at relatively
low temperature. A process for producing grain-oriented silicon steel at
medium temperature using aluminum nitride and copper as inhibitors may
realize the relatively low temperature for heating slab
(1250-1300° C.). This process adopts double cold rollings with
complete decarburizing annealing therebetween, wherein the complete
decarburizing annealing (to reduce carbon to below 30 ppm) is carried out
after the first cold rolling, and the resultant steel is rolled to the
thickness of steel sheet with the second cold rolling before it is coated
with MgO annealing separator as it is or after it is recovery annealed at
low temperature, followed by high-temperature annealing and post
treatment. In order to form complete glass film at the stage of
high-temperature annealing, the conditions for decarburizing annealing in
the process of heating slab at medium temperature have to be controlled
strictly to form an appropriate oxide layer on the surface. However, the
slab to be decarburizing annealed between the two cold rollings is rather
thick. Under the decarburizing annealing conditions which can ensure
formation of an appropriate oxide layer, carbon can not be reduced to
below 30 ppm. Furthermore, the oxide layer on the surface is damaged
during the second cold rolling after decarburizing annealing, throwing an
impact on the surface quality.

[0003] In the production of grain-oriented silicon steel, it has always
been difficult to form a good underlying layer that guarantees the
tension effect and the insulating effect of tension coating. However, the
unevenness at the joint of the underlying layer and the substrate may
hinder magnetic domain activity, leading to an increase of iron loss. On
the other hand, the existence of the glass film underlying layer results
in poor stamping performance of the grain-oriented silicon steel. In
order to further lower iron loss and improve stamping performance,
grain-oriented silicon steel without underlying layer has been developed
recently.

[0004] According to the method disclosed in Chinese Patent 03802019.X, the
composition of the slab based on mass comprises Si 0.8˜4.8%, C
0.003˜0.1%, acid soluble Al 0.012-0.05%, N 0.01% or less than
0.01%, balanced by Fe and unavailable inclusions. After hot rolling, the
resultant hot rolled sheet is formed to the final thickness of the sheet
via single cold rolling or two or more times of cold rollings with middle
annealing therebetween as it is or after it is annealed. Subsequently, in
an atmosphere with an oxidability that will not render formation of
oxides of Fe family, the steel sheet is subjected to decarburizing
annealing. After an oxide layer comprising silicon oxide as the main
component is formed on the steel sheet surface, an annealing separator
comprising aluminum oxide as the main component is coated to make a
mirror-like surface of the annealed steel sheet. Secondary
recrystallization is stabilized by controlling the moisture entrapped by
the annealing separator which comprises aluminum oxide as the main
component and is coated in the form of aqueous slurry and then dried, and
by controlling the partial pressure of vapor during annealing the steel
sheet.

[0005] According to the method disclosed in Korean Patent KR 526122,
decarburization and nitridation are carried out concurrently in a process
for producing silicon steel at low temperature, wherein magnesium oxide
separator added with SiO2 and Cl is used to avoid formation of an
underlying layer during high-temperature annealing. This method is
characterized by the following features. The composition of the billet
based on weight comprises C 0.045-0.062%, Si 2.9-3.4%, P 0.015-0.035%,
Als (acid soluble Al) 0.022-0.032%, Cu 0.012-0.021% N 0.006-0.009%, S
0.004-0.010%. The temperature at which the billet is heated is controlled
in the range of 1150-1190° C. After cold rolled to the thickness
of steel sheet, the steel sheet is decarburized and annealed at
840-890° C. in a protective atmosphere of wet nitrogen and
hydrogen containing ammonia. A separator comprising 100 parts by weight
of MgO+3-12 parts by weight of SiO2+25 parts by weight of chloride
ions as the main components is used for high-temperature annealing.

[0006] The above two patents are directed to grain-oriented silicon steel
without underlying layer. They both use (Al, Si) N or AlN+MnS as
inhibitors, and adopt a conventional high-temperature or low-temperature
production process in which the billet is cold rolled to the thickness of
steel sheet before decarburizing annealing, for the purpose of further
lowering iron loss and improving stamping performance.

[0008] The object of the invention is to provide a method for producing
grain-oriented silicon steel containing copper, wherein no underlying
layer is formed during high-temperature annealing, and grain-oriented
silicon steel with superior electromagnetic performances and surface
quality is obtained.

[0009] The invention is realized by a process for producing grain-oriented
silicon steel containing copper, comprising:

[0010] Secondary refining and continuous casting of molten steel in a
converter or an electric furnace to obtain casting blank having the
following composition based on weight: C 0.010%-0.050%, Si 2.5%-4.0%, Mn
0.1%-0.30%, Als 0.006%-0.030%, Cu 0.4%-0.7%, N 0.006%-0.012%,
S≦0.025%, balanced by Fe and unavailable inclusions;

[0015] Applying tension coating on the surface of the steel sheet and
stretch-leveling annealing.

[0016] With respect to the high-temperature annealing process, the steel
sheet is coated with a high-temperature annealing separator in the form
of aqueous slurry after the secondary cold rolling and dried to reduce
the water content of the separator to less than 1.5%, or dry coated
directly by electrostatic coating; and then the steel sheet is
high-temperature annealed in a protective atmosphere comprising hydrogen,
wherein the oxidability (P.sub.H2O/P.sub.H2) of the protective atmosphere
is in the range of 0.0001-0.2.

[0017] The main component of the high-temperature annealing separator is
selected from any one of zirconia ceramic fine powder, alumina fine
powder and silicon dioxide fine powder, or a combination of any two or
three of zirconia ceramic fine powder, alumina fine powder and silicon
dioxide fine powder.

[0018] The hot rolling, cold rolling and other processes in the invention
are conventional technical means in the art. Specifically, the hot
rolling is carried out by heating a slab in a heating furnace to above
1250° C. and holding this temperature for over 2 hours. It should
be ensured that the rolling begins at 1050-1200° C., preferably
1070-1130° C., and ends at above 800° C., preferably above
850° C. The slab is finally rolled into a hot rolled sheet of
2.0-2.8 mm in thickness.

[0019] After the hot rolling, the resultant hot rolled sheet is acid
washed, subjected to the primary cold rolling to obtain a medium
thickness of 0.50-0.70 mm, and then degreased.

[0020] Subsequently, the decarburizing annealing and the secondary cold
rolling are carried out. After the secondary cold rolling, the thickness
of the steel sheet is 0.15-0.50 mm. And then, the steel sheet is
degreased, annealed at high temperature, coated with the tension coating
and stretch-leveling annealed.

[0021] According to the invention, any one of zirconia ceramic fine
powder, alumina fine powder and silicon dioxide fine powder, or a
combination of any two or three of zirconia ceramic fine powder, alumina
fine powder and silicon dioxide fine powder is used as the main separator
which does not react with the surface oxides during high-temperature
annealing. The high-temperature annealing atmosphere is strictly
controlled to allow reduction of the surface oxides at the stage of
high-temperature annealing, wherein the surface oxides formed during
decarburizing annealing comprise SiO2 as the main component. Thus, a
mirror-like final product without glass film is formed. After applying
the tension coating, grain-oriented silicon steel with superior surface
quality and magnetic performance is obtained. The method of the invention
has thoroughly solved the problems such as unsteady quality, easy peeling
of the surface coating, unconspicuous tension effect, poor insulation and
surface quality that are encountered in conventional processes for
heating slab at medium temperature.

[0022] The invention exhibits the following beneficial effects:

[0023] Since no glass film is formed during high-temperature annealing
according to the method of the invention for producing grain-oriented
silicon steel containing copper, decarburizing annealing atmosphere
needn't to be strictly controlled to avoid formation of iron oxides. In
other words, middle decarburizing annealing may be carried out at a
relatively high oxidability (P.sub.H2O/P.sub.H2). Therefore, it may be
ensured that the carbon content is lowered to 30 ppm or less due to an
increase of decarburizing efficiency. Degradation of magnetic performance
due to magnetic aging of the final product is thus avoided. On the other
hand, the productive efficiency may be enhanced for the time of the
middle decarburizing annealing is shortened.

[0024] According to the invention, shot blasting and acid washing are
carried out after middle decarburizing annealing to remove the oxide
layer comprising mainly iron oxides from the surface, and thus improve
the surface quality of the slab after secondary cold rolling and that of
the final product effectively. Since the separator is directly coated
after secondary cold rolling to carry out high-temperature annealing, no
recovery annealing is needed, so that problems such as degradation of
magnetic performance and instability of the underlying layer are avoided,
and productive efficiency is enhanced.

[0025] Since no underlying layer is formed during the high-temperature
annealing according to the invention, there is no need to control the
composition of the separator and the coating modes strictly, so that the
production stability is enhanced and the purifying effect of steel is
improved effectively. A mirror-like final product is obtained, which has
no oxide layer on the steel sheet surface and unevenness of the glass
film that hinder magnetic domains from moving. Therefore, iron loss is
decreased significantly.

[0026] In summary, the invention provides a method for producing
grain-oriented silicon steel sheet with low cost, high efficiency and
good feasibility, which not only inherits the advantages of heating slab
at medium temperature, but also effectively solves the problems such as
insufficient decarburization, degradation of magnetic performance due to
recovery annealing, poor adhesion of the coating, unconspicuous tension
effect and poor surface quality that exist in the process for heating
slab at medium temperature.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

[0027] Steel was smelted in a 500 kg vacuum furnace. The chemical
composition (wt %) of the slab comprised 0.035% C, 3.05% Si, 0.020% S,
0.008% Als, 0.0010% N, 0.60% Cu, 0.15% Mn, balanced by Fe and unavailable
inclusions. The slab of this composition was hot rolled by heating it to
1280° C. and holding this temperature for 3 hours. The rolling was
ended at 930-950° C. After rolling, the resultant steel was cooled
by laminar flow, and then coiled at 550° C.±30° C. to
form band steel of 2.5 mm in thickness. After shot blasting and acid
washing, the band steel was cold rolled to a thickness of 0.65 mm and
then subjected to middle annealing to reduce carbon to 30 ppm or less.
After shot blasting and acid washing, three processes are carried out
respectively.

[0028] (1) The band steel was subjected to secondary cold rolling to 0.30
mm, the thickness of the final product, coated with an annealing
separator comprising Al2O3 slurry as the main component and
dried. Thereafter, the steel band was coiled and subjected to
high-temperature annealing in an atmosphere of mixed nitrogen and
hydrogen or pure hydrogen at 1200° C. which was held for 20 hours.
After uncoiled, the steel band was coated with insulating coating and
stretch-leveling annealed.

[0029] (2) The band steel was subjected to secondary cold rolling to 0.30
mm, the thickness of the final product, coated with an annealing
separator comprising MgO as the main component. Thereafter, the steel
band was coiled and subjected to high-temperature annealing in an
atmosphere of mixed nitrogen and hydrogen or pure hydrogen at
1200° C. which was held for 20 hours. After uncoiled, the steel
band was coated with insulating coating and stretch-leveling annealed.

[0030] (3) The band steel was subjected to secondary cold rolling to 0.30
mm, the thickness of the final product, annealed at 700° C. in a
wet atmosphere of nitrogen and hydrogen, coated with an annealing
separator comprising MgO as the main component. Thereafter, the steel
band was coiled and subjected to high-temperature annealing in an
atmosphere of mixed nitrogen and hydrogen or pure hydrogen at
1200° C. which was held for 20 hours. After uncoiled, the steel
band was coated with insulating coating and stretch-leveling annealed.

[0031] The magnetic and coating performances of the resultant products are
shown in Table 1.

[0032] Steel was smelted in a 500 kg vacuum furnace. The chemical
composition (wt %) of the slab comprised 0.032% C, 3.15% Si, 0.016% S,
0.012% Als, 0.0092% N, 0.48% Cu, 0.20% Mn, balanced by Fe and unavailable
inclusions. The slab of this composition was hot rolled by heating it to
1280° C. and holding this temperature for 3 hours. The rolling was
ended at 930-950° C. After rolling, the resultant steel was cooled
by laminar flow, and then coiled at 550° C.±30° C. to
form band steel of 2.5 mm in thickness. After shot blasting and acid
washing, the band steel was cold rolled to a thickness of 0.65 mm and
then subjected to middle annealing at 850° C. under the conditions
given in Table 2. After shot blasting and acid washing, the band steel
was subjected to secondary cold rolling to 0.30 mm, the thickness of the
final product, coated with an annealing separator comprising
Al2O3 slurry as the main component and dried. Thereafter, the
steel band was coiled and subjected to high-temperature annealing in an
atmosphere of mixed nitrogen and hydrogen or pure hydrogen at
1200° C. which was held for 20 hours. After uncoiled, the steel
band was coated with insulating coating and stretch-leveling annealed.
The magnetic and coating performances of the resultant products are shown
in Table 2, wherein the adhesion was evaluated according to the method
and standard defined in National Standards GB/T 2522-1988.

[0033] Steel was smelted in a 500 kg vacuum furnace. The chemical
composition (wt %) of the slab comprised 0.032% C, 3.15% Si, 0.016% S,
0.012% Als, 0.0092% N, 0.48% Cu, 0.20% Mn, balanced by Fe and unavailable
inclusions. The slab of this composition was hot rolled by heating it to
1280° C. and holding this temperature for 3 hours. The rolling was
ended at 930-950° C. After rolling, the resultant steel was cooled
by laminar flow, and then coiled at 550° C.±30° C. to
form band steel of 2.5 mm in thickness. After shot blasting and acid
washing, the band steel was cold rolled to a thickness of 0.65 mm and
then subjected to middle annealing at 850° C. under the conditions
given in Table 3. After shot blasting and acid washing, the band steel
was subjected to secondary cold rolling to 0.30 mm, the thickness of the
final product, coated with an annealing separator comprising
Al2O3 slurry as the main component and dried. Thereafter, the
steel band was coiled and subjected to high-temperature annealing in an
atmosphere of mixed nitrogen and hydrogen or pure hydrogen at
1200° C. which was held for 20 hours. After uncoiled, the steel
band was coated with insulating coating and stretch-leveling annealed.
The magnetic and coating performances of the resultant products are shown
in Table 3.

[0034] Steel was smelted in a 500 kg vacuum furnace. The chemical
composition (wt %) of the slab comprised 0.032% C, 3.15% Si, 0.016% S,
0.012% Als, 0.0092% N, 0.48% Cu, 0.20% Mn, balanced by Fe and unavailable
inclusions. The slab of this composition was hot rolled by heating it to
1280° C. and holding this temperature for 3 hours. The rolling was
ended at 930-950° C. After rolling, the resultant steel was cooled
by laminar flow, and then coiled at 550° C.±30° C. to
form band steel of 2.5 mm in thickness. After shot blasting and acid
washing, the band steel was cold rolled to a thickness of 0.65 mm and
then subjected to middle annealing at 850° C. under the conditions
given in Table 4. After shot blasting and acid washing, the band steel
was subjected to secondary cold rolling to 0.30 mm, the thickness of the
final product, electrostatically coated with an annealing separator
comprising Al2O3 as the main component. Thereafter, the steel
band was coiled and subjected to high-temperature annealing in an
atmosphere of mixed nitrogen and hydrogen or pure hydrogen at
1200° C. which was held for 20 hours. After uncoiled, the steel
band was coated with insulating coating and stretch-leveling annealed.
The magnetic and coating performances of the resultant products are shown
in Table 4.

[0035] Steel was smelted in a 500 kg vacuum furnace. The chemical
composition (wt %) of the slab comprised 0.032% C, 3.15% Si, 0.016% S,
0.012% Als, 0.0092% N, 0.48% Cu, 0.20% Mn, balanced by Fe and unavailable
inclusions. The slab of this composition was hot rolled by heating it to
1280° C. and holding this temperature for 3 hours. The rolling was
ended at 930-950° C. After rolling, the resultant steel was cooled
by laminar flow, and then coiled at 550° C.±30° C. to
form band steel of 2.5 mm in thickness. After shot blasting and acid
washing, the band steel was cold rolled to a thickness of 0.65 mm and
then subjected to middle annealing at 850° C. under the conditions
given in Table 5. After shot blasting and acid washing, the band steel
was subjected to secondary cold rolling to 0.30 mm, the thickness of the
final product, coated with an annealing separator comprising ZrO2
slurry as the main component and dried or electrostatically coated
directly with an annealing separator comprising ZrO2 fine powder as
the main component. Thereafter, the steel band was coiled and subjected
to high-temperature annealing in an atmosphere of mixed nitrogen and
hydrogen or pure hydrogen at 1200° C. which was held for 20 hours.
After uncoiled, the steel band was coated with insulating coating and
stretch-leveling annealed. The magnetic and coating performances of the
resultant products are shown in Table 5.

[0036] According to the invention which inherits the advantages of heating
slab at medium temperature, the process in which no underlying layer is
formed during high-temperature annealing is utilized, and the
decarburizing annealing process and the high-temperature annealing
process are controlled strictly, so that mirror-like grain-oriented
silicon steel without underlying layer is obtained. The final product
with tension coating has good appearance and electromagnetic
characteristics, and enhanced stamping performance. The method of the
invention has reduced procedures and enhanced productive efficiency, and
produces products with stable performances. The devices used herein are
conventional devices for producing grain-oriented silicon steel, wherein
the technologies and control means are simple and practical.